Gas turbine combustion systems utilizing can type combustors are often prone to air flow mal-distribution. The problems caused by such anomalies are of particular concern in the development of low NOx systems. The achievement of low levels of oxides of nitrogen in combustors is closely related to flame temperature and its variation through the early parts of the reaction zone. Flame temperature is a function of the effective fuel-air ratio in the reaction zone which depends on the applied fuel-air ratio and the degree of mixing achieved before the flame front. These factors are obviously influenced by the local application of fuel and associated air and the effectiveness of mixing. Uniform application of fuel typically is under control in well designed injection systems but the local variation of air flow is often not, unless special consideration is given to correct mal-distribution.
The achievement of current levels of oxides of nitrogen set by regulations in some areas of the world calls for effective fuel-air ratio to be controlled to low standard deviations on the order of 10%. The cost of development of such combustion systems is high but can be significantly influenced by the right choice of configuration. However, the use of film cooling in these low flame temperature combustors generates high levels of carbon monoxide emissions. External impingement cooling of the flame tube (liner) can curtail such high levels. Moreover, in systems where high exit temperature is a performance requirement in addition to low NOx, the air flow to swirler/reaction zone is a large proportion of total air flow and therefore cooling and dilution air flows are limited. Hence there is considerable advantage in controlling these flows to optimize the overall flow conditions.
One such recent combustor design is that shown in U.S. Pat. No. 7,617,684 to Norster, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. In the subject Norster combustor, essentially all the air flow for combustion is first separated from the dilution air stream and used for impingement cooling the portion of a combustor liner defining the combustion zone, and then channeled to swirl vanes for mixing with fuel. While the features of the Norster combustor may provide better control of the amount of air delivered to the swirl vanes, and thus the bulk fuel/air ratio, compared to previous impingement cooled combustors, further improvements in the aerodynamics of the combustion air flow to the swirl vanes may minimize local deviations in the fuel/air ratio. Improvements are also possible in the control of other cooling air flows in the combustor, which affect the level of emissions and the thermal efficiency of the combustor. Such improvements are set forth hereinafter.